Is Your Hydraulic Fluid Contamination Control Strategy Balanced?

By Mark Barnes, Senior Vice President, Senior Lubrication SME for Des-Case.

Particle contamination is the leading cause of hydraulic system failure, affecting both pump and valve life. It has been estimated that as many as 80-90% of hydraulic system failures can be attributed to contamination of hydraulic fluid. For this reason, hydraulic system designers always include filtration on the high-pressure supply line and, depending on the system design, sometimes on the return line. 

But how do you know if the system is clean, and how can you ensure that the system remains clean? The answer is simple: balanced contamination control. 

In the context of fluid contamination, balance means ensuring that the rate of contamination removal through filtration and other means meets or exceeds the rate of ingression (Figure 1). If more contamination enters the system per minute than can be effectively removed by the system filters, the overall hydraulic system will be out of balance and will inevitably become dirtier. Conversely, if the ingression rate is low enough and/or the removal rate is high enough, the system will become and remain balanced and clean. 

While the concept of balanced filtration may seem simple, attaining and retaining balance is not always easy. Over time, as vents and breathers plug or filters start to load with particles or moisture, systems that start in balance may fall out of balance. Maintaining balance is thus an ongoing and dynamic task, requiring careful monitoring of the condition of vents, breathers, and seals to maintain low ingression rates. It also calls for monitoring filter differential pressures to ensure filters remain working at optimum efficiency.

But how do you know if the system is balanced in the first place? 

The first step in achieving balance is to understand the overall system design and how the system will be utilized. For example, if a hydraulic power unit is to be used to power a hydraulic ram or press, each stroke of the press will effectively displace the same amount of air as the volume of oil used to fill or drain the hydraulic cylinders. If the press is large, this means that a significant number of cubic feet or meters of air will be exchanged with the ambient environment with each cycle of the press. As such, if the press is operating in a dusty or humid environment, the ingression rate will be far higher, meaning that a greater focus needs to be placed on optimizing contamination exclusion. This needs to be done while increasing removal rates to achieve or maintain balance.

By contrast, if the hydraulic system is powering a hydraulic motor or double-acting cylinder, fluid levels in the system will not change as much during normal operations. In this case, air will still be exchanged with the ambient environment through thermal siphoning to achieve normal heating/cooling during operations. However, the ingression rate will be much lower, meaning it will be easier to maintain balance.

Likewise, the cycle rate can impact how balanced a system is. A system that actuates 3 times per minute will have an ingression rate 3 times greater than a system that cycles once per minute and will require a much higher rate of removal to maintain balance.

Strategies for Achieving Balance

Once we understand how the system will be utilized, we need to look at how we intend to maintain balance. In most circumstances, the best place to start is to ensure that the ingression rate is as low as possible. This means looking at the overall system design to understand all possible sources of ingression. 

While there are several different ingression points in most systems, one of the most common, and often the leading, source of ingression is the air vent or breather.

Many small-to-medium-sized hydraulic units are designed with a combination of fill port and air vents (Figure 2). These often contain little more than foam, wire wool, or a mesh screen, none of which offer any protection against the silt-sized particles in the 1-10 micron range that cause most of the issues in hydraulic systems. They also do nothing to exclude airborne moisture or humidity. 

In this case, changing the filler/breather cap to a desiccant breather that removes particles as small as 1 micron, as well as water, is an inexpensive upgrade that will significantly reduce the ingression rate.

In addition to upgrading the breather, we need to also look at the reservoir as a whole to ensure that all inspection points are properly closed and sealed with a gasket.

Another common source of ingression is from new oil additions and top-offs. Since most new oils are not clean to the level that contamination-sensitive hydraulic systems demand, all new hydraulic fluid should be pre-filtered before adding to the system. 

The method of oil addition can also have an impact on particle ingression rates. Removing the fill port or topping off with an open container can have a significant impact on contamination loading. This is particularly true in dusty, humid operating conditions. 

The simplest way to address this is through the use of quick connects. By replacing the standard filler/breather port with a manifold that allows for both a desiccant breather and quick connect to be installed, the system can remain sealed and isolated during every phase of maintenance and operations. This helps to reduce ingression rates and achieve balanced contamination control (Figure 2).

Once every means of reducing contamination ingression has been evaluated and optimized, our focus then needs to turn to filtration.

Full-flow filters should be carefully selected to help balance contamination control. If our goal is to achieve a very stringent level of cleanliness such as an ISO 15/13/10 fluid cleanliness, demanded by high-pressure systems with servo valves, our filters need to match that level of rigor. While this is true, it is not uncommon to see 10-micron filters being used on hydraulics systems that are completely out of balance. If our goal is to remove 1-10 micron particles, our filters need to be at least β≥200 at 3 µm or better.

In very dusty or dirty environments, we may need to increase the capture efficiency or reduce the micron size rating of the filter even further to capture a larger number of particles to offset higher ingression rates and maintain balance.

While evaluating and upgrading full-flow systems, filters are always a good place to start. In some circumstances, it is not practical and/or prohibitively expensive to increase filter efficiency without impacting flow rates and cycle times. Under these circumstances, consideration should be given to adding supplemental filtration in the form of offline or kidney loop filtration.

Offline filtration

Offline filters are typically installed on the reservoir and designed with their own motor, pump, and filtration system (Figure 3). By pulling a small amount of oil from the bottom of the reservoir, passing it through a very fine filter (often with a β-rating above 2000 at 2 microns), and returning it to the reservoir as far from the inlet line as practical, offline filtration is an excellent complement to existing filters. Doing this allows a much higher rate of contamination removal, helping to maintain balance.

Offline systems should be selected to pass the entire volume of the reservoir between 5-10 times in 24 hours. Where higher ingression rates are anticipated, increasing the flow rate to as much as 1 pass per hour may be required to maintain balance. 

Unlike full-flow filters, offline filters can be left running even if the system is shut down or idle, helping to keep the oil both clean and warm, which can have a significant impact on reducing both condensation build-up and varnish deposit formation.

Where moisture ingression is an issue, a water-absorbing pre-filter can be used to eliminate water ingression through wiper seals and other ingression points.

In mobile hydraulic systems, ingression rates for contaminants are often orders of magnitude higher than in fixed plant systems. This is due to the constant cycling of hydraulic cylinders and the dusty environment found in most off-highway work sites. Under these circumstances, bypass filtration can be installed directly into the pressure side of the system through a pilot circuit. Bypass units do not require an additional pump or motor; they function by using system pressure to force oil to return to the reservoir through an offline high-efficiency filter. By reducing system pressure to less than 40 psi through the use of a pressure-compensated flow control valve, low-pressure, high-efficiency radial filters can be used to reduce contamination loading and maintain balance (Figure 4).

The Importance of Oil Analysis

One of the most important aspects of maintaining balanced contamination control is monitoring fluid cleanliness via routine oil analysis. Hydraulic systems should be monitored at least every 30 days and in some circumstances even more frequently. In addition to routine oil analysis testing such as viscosity and wear metals, ISO particle counting should be standard.

The ISO 4406 fluid cleanliness standard establishes range codes such as 15/13/10 that represent contamination levels at 4, 6, and 14 microns per mL of fluid. Addressing balance requires looking at the prevailing ISO range codes, as well as the absolute number of particles, which should be trended over time.

Figure 5 shows the results of monthly particle counts from a high-pressure hydraulic system using piston pumps and servo valves. The system was equipped with full-flow filters from the OEM, rated at β=1000 at 10 microns. As can be seen from the graph, while the 10 µm filters can reduce and create balanced filtration at 10 microns and above, the system is still heavily contaminated with 4 and 6-micron particles which, particularly at 4 microns are increasing over time. This is a clear indication that this system is out of balance in the silt-sized particle range (<10 microns).

Based on the typical clearances in high-pressure servo valves, which are usually less than 3-4 microns, it is unlikely this system will achieve optimum reliability without either reducing the ingression rate or increasing the particle removal rate.

Summary

Balanced filtration is a simple concept: ensure that the amount of contamination being removed by system filters over time meets or exceeds the rate of ingression. It starts with lowering ingression rates through proper reservoir, seal, and breather management, as well as looking for opportunities to increase removal rates through upgraded full-flow, offline or bypass filtration. By using oil analysis and, in particular, particle count trends, balance can be achieved and maintained. However, it cannot be assumed that a hydraulic system will be balanced just because the system has a filter supplied by the OEM that was periodically replaced.